Generation and characterization of microwave quantum states

Quantum mechanics is the branch of physics that describes the properties and behavior of systems on the atomic and subatomic level. Over the past decades there has also been considerable progress in engineering larger-scale quantum systems. In this day and age, quantum information and quantum technology are rapidly developing areas of research where quantum effects are harnessed to improve sensitivity in measurements, encrypt secure communications, and enhance the performance of information processing and computing. Specific types of quantum states are needed for these purposes, and they can be challenging to generate in practice. This thesis describes methods to generate and characterize microwave states that could be useful for quantum computing protocols based on quantum states of light

Simple, Reliable, and Noise-Resilient Continuous-Variable Quantum State Tomography with Convex Optimization

Precise reconstruction of unknown quantum states from measurement data, a process commonly called quantum state tomography, is a crucial component in the development of quantum informa-tion processing technologies. Many different tomography methods have been proposed over the years. Maximum-likelihood estimation is a prominent example, being the most popular method for a long period of time. Recently, more advanced neural-network methods have started to emerge. Here, we go back to basics and present a method for continuous-variable state reconstruction that is both conceptually and prac-tically simple, based on convex optimization. Convex optimization has been used for process tomography and qubit-state tomography, but seems to have been overlooked for continuous-variable quantum-state tomography. We demonstrate high-fidelity reconstruction of an underlying state from data corrupted by thermal noise and imperfect detection, for both homodyne and heterodyne measurements. A major advan-tage over other methods is that convex optimization algorithms are guaranteed to converge to the optimal solution

Realistic prospect for continuous variable quantum computing in circuit-QED

This licentiate thesis is an extended introduction to the appended papers, which pertain to finding quantum states that are useful for continuous variable quantum computing. The useful states are characterized by a negative Wigner function. This is the underlying motivation for the appended papers, but why a negative Wigner function is necessary is not explained in the papers. This is elucidated in this thesis, with an accompanying discussion of which quantum mechanical properties allow quantum computers to surpass the capabilities of classical computers

Wigner negativity in the steady-state output of a Kerr parametric oscillator

The output field from a continuously driven linear parametric oscillator may exhibit considerably more squeezing than the intracavity field. Inspired by this fact, we explore the nonclassical features of the steady-state output field of a driven nonlinear Kerr parametric oscillator using a temporal wave packet mode description. Utilizing a new numerical method, we have access to the density matrix of arbitrary wave packet modes. Remarkably, we find that even though the steady-state cavity field is always characterized by a positive Wigner function, the output may exhibit Wigner negativity, depending on the properties of the selected mode

Numerical study of Wigner negativity in one-dimensional steady-state resonance fluorescence

In a numerical study, we investigate the steady-state generation of nonclassical states of light from a coherently driven two-level atom in a one-dimensional waveguide. Specifically, we look for states with a negative Wigner function, since such nonclassical states are a resource for quantum information processing applications, including quantum computing. We find that a waveguide terminated by a mirror at the position of the atom can provide Wigner-negative states, while an infinite waveguide yields strictly positive Wigner functions. Moreover, our paper reveals a connection between the purity of a quantum state and its Wigner negativity. We also analyze the effects of decoherence on the negativity of a state

Norwegian nursing students' evaluation of vSim® for Nursing

Background: vSim® for Nursing is the first web-based platform linked to the nursing education curriculum. It is an American simulation tool, developed in 2014 through a collaboration between Wolters Kluwer Health, Laerdal Medical and the National League for Nursing. To our knowledge, no studies have evaluated vSim® for Nursing from the nursing students’ perspective in Norway. The aim of the study was to evaluate second year Norwegian nursing students’ experiences with the virtual clinical simulation scenario in surgical nursing from vSim® for Nursing. Methods: A descriptive and a convergent mixed method design was utilised. The method comprised a 7-item questionnaire with five open-ended questions. Sixty-five nursing students participated in the study. Results: The majority of Norwegian nursing students evaluated the virtual clinical scenario in surgical nursing from vSim® for Nursing useful, realistic and educational in preparing for clinical placement in surgical care. However, a small portion of the nursing students had trouble understanding and navigating the American vSim® for Nursing program. Conclusions: Introducing virtual simulation tools into the nursing education encompasses faculty and student preparation, guidance from faculty members during the simulation session and support for students who are facing difficulties with the simulation program.publishedVersio

Steady-State Generation of Wigner-Negative States in One-Dimensional Resonance Fluorescence

In this work we demonstrate numerically that the nonlinearity provided by a continuously driven two-level system allows for the generation of Wigner-negative states of the electromagnetic field confined in one spatial dimension. Wigner-negative states, also known as Wigner nonclassical states, are desirable for quantum information protocols beyond the scope of classical computers. Focusing on the steady-state emission from the two-level system, we find the largest negativity at the drive strength where the coherent reflection vanishes

Engineering Symmetry-Selective Couplings of a Superconducting Artificial Molecule to Microwave Waveguides

Tailoring the decay rate of structured quantum emitters into their environment opens new avenues for nonlinear quantum optics, collective phenomena, and quantum communications. Here, we demonstrate a novel coupling scheme between an artificial molecule comprising two identical, strongly coupled transmon qubits and two microwave waveguides. In our scheme, the coupling is engineered so that transitions between states of the same (opposite) symmetry, with respect to the permutation operator, are predominantly coupled to one (the other) waveguide. The symmetry-based coupling selectivity, as quantified by the ratio of the coupling strengths, exceeds a factor of 30 for both waveguides in our device. In addition, we implement a Raman process activated by simultaneously driving both waveguides, and show that it can be used to coherently couple states of different symmetry in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency of about 95%. Finally, we show that this coupling arrangement makes it possible to straightforwardly generate spatially separated Bell states propagating across the waveguides. We envisage further applications to quantum thermodynamics, microwave photodetection, and photon-photon gates

Robust Preparation of Wigner-Negative States with Optimized SNAP-Displacement Sequences

Hosting nonclassical states of light in three-dimensional microwave cavities has emerged as a promising paradigm for continuous-variable quantum information processing. Here we experimentally demonstrate high-fidelity generation of a range of Wigner-negative states useful for quantum computation, such as Schrodinger-cat states, binomial states, Gottesman-Kitaev-Preskill states, as well as cubic phase states. The latter states have been long sought after in quantum optics and have never been achieved experimentally before. We use a sequence of interleaved selective number-dependent arbitrary phase (SNAP) gates and displacements. We optimize the state preparation in two steps. First we use a gradient-descent algorithm to optimize the parameters of the SNAP and displacement gates. Then we optimize the envelope of the pulses implementing the SNAP gates. Our results show that this way of creating highly nonclassical states in a harmonic oscillator is robust to fluctuations of the system parameters such as the qubit frequency and the dispersive shift